In an electron beam apparatus such as an SEM, as the diameter of the electron probe on a specimen is reduced, the specimen image is obtained with higher resolution and higher contrast. It is known that the diameter of the electron probe on the specimen assumes its minimum value when the convergence angle .alpha. of the probe takes a certain optimum value .alpha..sub.opt, and that the optimum value .alpha..sub.opt differs, depending on the accelerating voltage V.sub.a, on the probe current impinging on the specimen and on the spherical and chromatic aberration coefficients of the final lens. Therefore, whenever at least one of the accelerating voltages and the probe current is varied, the convergence angle is so adjusted as to assume the optimum value .alpha..sub.opt under the varied conditions.
One conventional method of adjusting the convergence angle is now described by referring to FIG. 1, where an electron gun 1 produces an electron beam 2 that is focused onto a specimen 5 via a condenser lens 3 and an objective lens 4. The condenser lens 3 and the objective lens 4 are provided with apertures 6 and 7, respectively. The convergence angle .alpha. can be changed by replacing the objective lens aperture 7 with an objective lens aperture having a different diameter.
Another conventional method of adjusting the convergence angle is now described by referring to FIG. 2, where an auxiliary lens 8 for adjusting the convergence angle is installed between a condenser lens 3 and an objective lens 4. An aperture 9 is installed in the principal plane of the auxiliary lens 8. The convergence angle can be varied by adjusting the strength of the auxiliary lens 8.
In the conventional method described already in connection with FIG. 1, the diameter of the aperture hole can be switched between several values, at best, practically. Therefore, it is impossible to accomplish the optimum convergence angle, depending on the values of the accelerating voltage and the probe current except for only several conditions.
In the method described in conjunction with FIG. 2, the aperture 9 is located in or close to the principal plane of the auxiliary lens and so, if the strength of the auxiliary lens 8 is changed, the amount of the electron passing through the aperture 9, i.e., the probe current, is varied by the lens effects of the magnetic field distributed upperside of the aperture. Consequently, it is impossible to vary only the convergence angle while maintaining the probe current constant to find out the optimum convergence angle .alpha..sub.opt. These situations for electron beam apparatus hold for ion beam apparatus, too. The conventional methods for electron beam apparatus illustrated in FIGS. 1 and 2 are disclosed in Japanese Patent Publication No. 10740/1981.